The present application relates to current-time protection systems and, more particularly, to a novel i.sup.2 t protection system capable of protecting active and passive electronic components against overloads.
All electronic and electric components, whether of active nature, e.g. a motor, or passive nature, e.g. wiring, switches and the like, have some finite dissipative quality which causes a temperature rise to occur within that component during use. Each component has a maximum rated temperature which is not to be exceeded during operation thereof. However, various operating conditions, e.g. due to inrush currents at the start of operation and the like, often cause a limited overload to occur. Such limited overload operation is allowable, as long as the maximum rated temperature of the component is not exceeded.
Assuming, for simplicity of explanation, a basically resistive component, the operating temperature thereof is determined by the equilibrium of the rate of cooling (W.sub.c), itself determined by the characteristics of the component determining the amount of heat energy transferred from that component to the ambient atmosphere and the rate of heating (W.sub.h) of the component due to the i.sup.2 r dissipative losses in the component. Assuming, again for the sake of simplicity, constant resistance r operation of the component in question, the rate of heating W.sub.h is given by ##EQU1## where K is a constant and i is the instantaneous current flowing through the component. For a component drawing a normal, or rated, instantaneous current i.sub.N, the normal rate of heating ##EQU2## For an overload current i.sub.OL flowing through the component, the overload heating rate ##EQU3## and a corresponding temperature rise results. On a transient basis, the temperature rise .DELTA.T during the presence of an overload current i.sub.OL is generally proportional to the additional dissipative energy and ##EQU4## If a maximum temperature rise .DELTA.T.sub.M is specified for a component, then the time t.sub.o (during which the overload current i.sub.OL flows) before the component is shut down to prevent damage, must be chosen as a function of the overload current itself.
The foregoing is commonly referred to as i.sup.2 t protection and is approximated within narrow ranges by a variety of devices such as fuses, thermal switches, electromagnetic switches and the like. However, even when a combination of these narrow-range devices are used, the resulting operation is often very different from an ideal i.sup.2 t relationship. Because each different protected component has a different required protective curve, it is often required to have a different protective combination associated with each component; for example, to protect a motor, a special "heater" unit is often used in the motor starter unit, with a different configuration, size and rating for the special "heater" unit utilized with each size and type of motor. It is clearly desirable to provide an i.sup.2 t protection system which not only provides an accurate representation of the i.sup.2 t function over an extended range, typically from a single cycle of the power line frequency (about 16 milliseconds for the 60 Hz. line frequency in use in the United States) to a time of at least 1 minute. It is also desirable to have a single i.sup.2 t protection system capable of properly protecting a wide range of components by means of varying the setting of simple adjustments provided therein.